How Sunflowers Follow The Sun

"In a study published in the journal Science this week, researchers say they've found an answer: Sunflowers, like animals, have a circadian rhythm — an internal clock that can be set to the external world. During the day, this system sends messages to the eastern sides of their stems, telling those cells to grow slightly longer, which causes the sunflower to lean westward. At night, the message reverses, and the sunflowers tilt back toward the east."
-The Washington Post

Which is pretty cool if you, and by that I mean me, have ever wondered about it.